Many of the voltage regulator modules (VRMs) used for high current application use synchronous rectification. FIG. 1a shows a typical non-isolated DC--DC converter VRM synchronous rectification circuit. The circuit includes first and second capacitors 12, 20, first and second transistors 14, 16 an inductor 18 and a pulse width modulator 20. In this type of configuration the first transistor 14 is referred to as the high switch and the second transistor 16 is referred to as the low switch. However, even though a synchronous mode of operation improves the efficiency of the DC--DC converter, it does not achieve an ideal Zero Volt Switch (ZVS) mode operation.
To better understand the operation of a typical synchronous rectification, refer to the wave form in FIG. 1b. VG1 illustrates the driving wave form of the first transistor 14 and VG2 shows the driving wave form of the second transistor 16. IQ1 is the transistor current of the first transistor 14 and IQ2 is the transistor current of the second transistor 16. Vds1 represents the drain to source voltage across the first transistor 14 and Vds2 represents the drain to source voltage across the second transistor 16.
Still referring to FIG. 1b, when the first transistor 14 is turned on at t.sub.0, the voltage across it, Vds1, is approximately equal to V.sub.in. Also, during the subsequent turn off of the first transistor 14 at t.sub.2, it experiences the full input voltage, Vin, while current is still flowing through it. Therefore, the first transistor 14, unlike the second transistor 16,does not turn on at a time when there is no voltage across it. Hence, it does not operate in ZVS mode. This creates switching losses which lowers the efficiency of the circuit.
This efficiency problem is typically addressed by utilizing an isolated synchronous rectification circuit. FIG. 2a is a schematic of an isolated synchronous rectifier 30. It includes a first capacitor 31 coupled to a first high frequency transformer 32, a winding 33 coupled to the transformer 32, a first transistor 34 coupled to the winding 33, a second transistor 36 coupled to the first transistor 34, an inductor 38 coupled to the second transistor 36 a capacitor 40 coupled to the inductor 38, a third transistor 42, a second transformer 44 and a pulse width modulator 46. By incorporating the transformer 32, an ideal Zero Volt Switch (ZVS) operation is achieved in both transistors 34, 36, wherein if the first transistor 34 is on, the second transistor 36 is off and only its body diode will be conducting.
Please refer now to FIG. 2b. VG1 illustrates the driving wave form of the first transistor 34 and VG2 shows the driving wave form of the second transistor 36. IQ1 is the transistor current of the first transistor 34 and IQ2 is the transistor current of the second transistor 36. Vds1 represents the drain to source voltage across the first transistor 34 and Vds2 represents the drain to source voltage across the second transistor 36. Unlike the non-isolated synchronous rectifier, when the first transistor 34 is turned on at t.sub.0, Vds1 is approximately zero volt. Furthermore, when the second transistor 36 is turned on at t.sub.2, Vds2 is approximately at zero volt. Accordingly, both the first transistor 34 and the second transistor 36 operate in ZVS mode.
This solution improves efficiency, but it is only beneficial in the operation of a single output synchronous rectifier where a single output voltage is desired. Ergo, this solution would not work in a multiple output circuit where several different output voltages are provided based upon one input voltage and a single isolate transformer. For example, a typical computer power supply system may require output voltages of +3.3V, +5V, +12V, etc. Consequently, in applications where different output voltages are generated from a single input voltage, the isolated synchronous rectifier circuit of FIG. 2 is not an effective solution. Accordingly, what is needed is an improved multiple output synchronous rectification circuit. The present invention addresses such a need.